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.NET Forum / Languages / Managed C++ / March 2005must go faster!
I am trying to figure out if I can use sse to help execute arithmetic operations faster. I have 900 values that must each be scaled with a divide and multiply. This happens repeatedly. Any examples I can be pointed to would be greatly appreciatted. I realize I could do just one multiply (instead of multiply and divide) but I still want to do 900 (or as many as I can) at once. Any ideas would be appreciatted. Bill Hi, If you post the algorithm, people may be able to help optimise it. We do a lot of intensive maths and use vector libraries (from Apple and Intel) to take care of the low-level stuff, like multiplication and division. We use the Intel Integrated Performance Primitives library and the performance is incredible (especially on Intel libraries) over doing hand-coded loops. That said, it may be possible to squeeze some performance out simply by optimising the algorithm (as you say, combining the multiplication and division). Steve >I am trying to figure out if I can use sse to help execute arithmetic >operations faster. I have 900 values that must each be scaled with a divide [quoted text clipped - 6 lines] > > Bill Ok here we go. A little mix of psuedocode and real code. For 900 blocks read signed integer values - there are 4 scale values ; scaled value = read value / 32768 * 360 store value as double next block read value is 16 bit signed int stored scaled value is of type double I realize I could just do : (double)round((double)read value/91.02222) But if I could do a vector, I could go fast. maybe do 900 at a time. I'm just not up on single instruction multiple data stuff. Just an example,,, please. Thanks, Bill > Hi, > [quoted text clipped - 43 lines] >> >> Bill Multiplication should be faster than division. Thus, instead of division by 91.0222, you can multiply by 0.010986328125 /Fredrik > Ok here we go. A little mix of psuedocode and real code. > [quoted text clipped - 64 lines] > >> > >> Bill I am new to MSC and .NET, coming form a Unix/Linux & Borland background. I am evaluating using MS tools instead of Borland tools for our PC/Windows apps. Anyhow, are you guys telling me that the expression "32768 * 360" is not optimized by the compiler? I thought all compilers would optimize all constant mathematical expressions. For documentation purposes I often use things like "60 * 60 * 24" to represent seconds in a day rather than just putting 86400. I have looked at the actual code generated and the constants have been evaluated to a single value, at least on other compilers... Back to the original question, I had some ideas about your original discussion: 1. Array libraries are great. It is often much quicker to look up complex math than to calculate it. 2. You didn't specify what you were doing with these numbers, or the degree of accuracy needed. Can you use binary math instead of real math? I mean there are several published routines that use binary approximations for things like sin(), cos() and others that are really fast, and the accuracy is good enough for calculating rotations of thing being displayed on the screen. 3. Again I was not sure if your example was exactly what you were trying to do, or just a short for instance, but there are a lot of shortcuts you can take if high precision is not necessary. Take a look at some of the math that gamers use to do quick calculations for drawing and scaling objects on the screen. They are not high precision, but they are FAST. >I am new to MSC and .NET, coming form a Unix/Linux & Borland background. I > am evaluating using MS tools instead of Borland tools for our PC/Windows [quoted text clipped - 7 lines] > constants have been evaluated to a single value, at least on other > compilers... Of course they are - I'm not sure how you reached that conclusion based on this thread, but... -cd > Of course they are - I'm not sure how you reached that conclusion > based on this thread, but... > > -cd From the statement "I realize I could do just one multiply (instead of multiply and divide) but" in the first message, and "Multiplication should be faster than division. Thus, instead of division by 91.0222, you can multiply by 0.010986328125" in the fourth message. I would not think that made any difference. I haven't looked at the machine code generated by the MS compiler yet, and probably won't. I just thought it sounded odd to be worrying about things like that or order of operators. My experience is that modern compilers generally optimize those things pretty well. The guys sounded like they were fairly familiar with math routines, I was just wondering why they were worrying about something that I thought was insignificant. 20 years ago programmers had to be concerned about the small details. Good programmers often took time to "optimize" their code for the compiler, manually align structures on word boundaries, etc... Aren't modern compilers wonderful? it has been a long time since i looked at the details, but division is generally slower than multiplication. the reason is that the algorithms involved in division are more complex than the routines for multiplication. the sort of optimization you talk about is done only with integer operations (at least that is my experience with the TI compiler for DSPs). the reason is that most floating point math is impossible to convert to a 100 % equivalent result if decimal values are used. for example, in my naive programming newbie years i had a piece of code like this: current = 0; while(current != 1.2) { //do stuff current+= 0.3; } can you guess what happened: right the loop never finished. the reason for this is that 4 times 0.3 is not equal to 1.2 (check floating point definition if you don't believe me). floating point math is not simple, and there are a number of rules you have to follow if you want to have accurate results. multiplying with the reverse of a number is not 100% the same as doing a division. it would be inexcusable if a compiler changed the functionality of a program without guaranteeing the same results. dividing an integer by 2 however is often replaced by shifting the bits 1 position to the right, because it is faster and it has the exact same result. kind regards, Bruno. >> Of course they are - I'm not sure how you reached that conclusion >> based on this thread, but... [quoted text clipped - 20 lines] > > Aren't modern compilers wonderful? <snip> >dividing an integer by 2 however is often replaced by shifting the bits 1 >position to the right, because it is faster and it has the exact same >result. In standard C and C++, this is necessarily true only for unsigned values! The result of right-shifting a signed quantity is implementation-defined. -- Sev Hi, Like Fredrick said, you can make that a multiplication. Vectorised code is useful for doing (like we do here) multiple iterations of exponentials, logs, fourier transforms etc over 65000 digit chunks. For 900 blocks I'd expect this kind of thing to be practically instantaneous on a modern processor. Steve > Ok here we go. A little mix of psuedocode and real code. > [quoted text clipped - 64 lines] >>> >>> Bill did you try compile with SSE and SSE2 (with Visual C++ 6.0/processor pack or Visual Studio .Net) ? pehaps this can help. This is just an idea... did you try compile with SSE and SSE2 (with Visual C++ 6.0/processor pack or Visual Studio .Net) ? pehaps this can help. This is just an idea... Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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